1. Field of the Invention
The invention relates to a photoacoustic multipass cell in which the excitation light is provided by a divergent light source and is reflected multiple times. The divergent light source is generally formed by the suitably shaped end of an optical fiber.
2. Background Description
Photoacoustic measuring methods, i.e., measuring methods in which the substance to be investigated, preferably a gas, is irradiated with a light source and heated by absorption, are highly suitable for precisely measuring the concentration of absorbent gases or absorbent substances in gases. The gas expands when it is heated. If heating, and hence, expansion are periodic, a sound wave forms that can be measured by a sound pressure sensor. Photoacoustic spectroscopy has numerous advantages over classic absorption spectroscopy in which the light passing through the sample is measured, and the absorption is determined from the difference between the incident light and the light passing through the sample. The photoacoustic signal is linear within a concentration range of approximately 5-6 orders of magnitude. Sensitive photoacoustic cells can also be realized by using optical cells known from the prior art. Light is thereby generated in a light source and introduced into the optical cell. Usually, the light source is outside, and the light is conducted through an optical fiber into the optical cell. By suitable optical elements, the light can be guided into the optical cell as a parallel beam or focused beam. Cells known from absorption spectroscopy can be used as the sensitive optical absorption cells. The light is thus generally reflected several times in the optical cell. The multiple passage of the light allows the light to be absorbed well by the absorbent substances in the cell. Finally, the light leaves the cell and reaches a detector in which the light intensity is measured. When the light path is known, the absorption can be inferred using the Beer-Lambert law by comparing the incident light intensity and the intensity after light has passed through the cell several times. If an optical multipass cell of this type is modified such that the optical detector is omitted and sound pressure sensors are arranged instead, a photoacoustic multipass cell is achieved. A disadvantage of photoacoustic multipass cells of this type, however, is their relatively low sensitivity due to a high volume; generally, the volume is at least one-half liter. High sensitivity can be achieved with a photoacoustic multipass cell with a concave mirror installed. Experiments and test measurements of a multipass cell have shown, however, that this construction makes extremely high demands on the precise angle of incidence of the laser radiation and the adjustment of the concave mirror. This is described by A. Miklos, J. Ng, P. Hess, A. H. Kung in “Application of a wavelength-amplitude double-modulation method in photoacoustic detection using a pulsed optical parametric oscillator,” Journal de Physique IV, 125, 579-582, (2005) and A. Miklos, S-C. Pei and A. H. Kung in the “Multipass acoustically open photoacoustic detector for trace gas measurements,” Applied Optics 45, 2529-2534, (2006).